ABSTRACT Influenza virus is one of the most important human infectious diseases. The influenza mortality is estimated to be approximately 650,000 per year worldwide, in addition, occasional global pandemics can infect up to 20- 40% of the world's population. Licensed influenza virus vaccines require annual reformulation and readministration due to poor IgG longevity and lack of neutralization of related viruses. We have developed a new vaccine approach against influenza virus that uses antigen-encoding nucleoside-modified mRNA encapsulated in lipid nanoparticles (LNPs). This vaccine format composed entirely of physiologic components that is simple and cost effective to produce and should induce no adverse events after administration. Additionally, mRNA vaccine production is sequence-independent and does not require eggs, cell culture or protein purification. We demonstrated that monovalent mRNA-LNP vaccines encoding full-length hemagglutinin (HA) induced potent immune responses against the immunosubdominant HA stalk region, one of the targets of universal influenza virus vaccines, which was associated with protection against homologous, heterologous and heterosubtypic influenza viruses in mice. Furthermore, we showed that a single immunization with nucleoside-modified mRNA-LNP vaccines induced durable protective HA stalk-specific antibody responses. Finally, we demonstrated that nucleoside-modified mRNA-LNP immunization induced high levels of antigen-specific T follicular helper (Tfh) cells that are critical for the generation of germinal centers, immunoglobulin class switch, and long-term memory, which are vital elements in the development of an effective influenza vaccine. In this grant, we aim to develop and evaluate new generation, broadly protective influenza virus vaccines in mice and ferrets using nucleoside-modified mRNA-LNPs encoding hemagglutinin, neuraminidase, nucleoprotein and the ectodomain of M2. Additionally, we aim to investigate the mechanisms of action of mRNA-LNP influenza virus vaccines. We will have 3 specific aims: 1) Design and production of mRNA immunogens for conserved regions of influenza virus. 2) Immunogenicity and protective efficacy of nucleoside-modified mRNA-LNP influenza virus vaccines in mice and ferrets. 3) Investigation of the mechanisms of action of nucleoside-modified mRNA-LNP influenza vaccines. This proposal will develop safe, new generation influenza virus vaccines that are easy to manufacture and administer and can protect against antigenically distant influenza viruses. The data generated will be capable of moving this vaccine approach to clinical trial development.